The present invention relates to the field of radio communications, and in particular to the techniques making it possible to form various types of dedicated channels for transmitting radio signals destined for a given station.
The invention finds application in particular in third-generation cellular networks of the UMTS type (xe2x80x9cUniversal Mobile Telecommunication Systemxe2x80x9d) using code-division multiple access (CDMA) techniques.
The fact that the dedicated signaling information and the traffic (voice or data) are transmitted over the radio interface with the same degree of immunity to noise is a limitation of certain radio communication systems. The radio coverage must be at least as great for the signaling as for the traffic. Otherwise, undesirable situations may occur, such as the inability of a subscriber to put an end to a communication in progress, or the inability to execute a cell transfer (xe2x80x9chandoverxe2x80x9d), etc.
Furthermore, in certain cases, such as for example when an adaptive multi-rate (AMR) codec is used to code speech, the immunity to noise may be enhanced by increasing the redundancy introduced by the channel coding and by correspondingly reducing the instantaneous bit rate of the source coder. In such cases, it is also desirable to be able to increase the noise immunity of the dedicated signaling.
U.S. Pat. No. 5,230,082 tackles the above problem, as regards the risk of inability to execute a handover when it is no longer possible for a mobile terminal to pick up the signaling information controlling such handover. The document proposes a mechanism through which a base station neighboring the one which was previously serving the mobile terminal borrows the physical communication resource to deliver the handover command to the terminal. This mechanism lacks flexibility and requires cooperation between the base stations as well as a dynamic scheme for radio resource allocation in the network infrastructure.
Moreover, a user data flow can contain control information mixed with the traffic, in particular control information from the higher layers of the OSI model (network, transport or application). Here again, it may be sensible to protect the control or signaling information more than the traffic data, which is not allowed by the current systems.
An object of the present invention is to propose a method meeting the above requirements.
The invention thus proposes a method of transmitting radio signals based on at least one data flow toward a radio communication station, comprising the steps of:
processing separately, in a radio link control stage first data units containing information of a specified type and second data units not containing information of the specified type;
supplying the data units to a medium access control stage which delivers the first data units along a first dedicated transport channel and the second data units along at least one second dedicated transport channel;
supplying the data units delivered along said dedicated transport channels to a coding and multiplexing stage to form at least one symbol stream pertaining to a dedicated physical channel; and
supplying each symbol stream to a radio transmission stage,
and wherein the coding and multiplexing stage and/or the radio transmission stage are controlled to provide greater protection against noise along the first dedicated transport channel than along the second dedicated transport channel.
The information of said specified type is preferably information pertaining to a signaling protocol, whereas the xe2x80x9csecond data unitsxe2x80x9d rather contain user data.
Several processes may be used to differentiate between the anti-noise protections afforded to the various transport channels. The coding and multiplexing stage can thus be controlled to apply a channel coding exhibiting a higher redundancy in the first dedicated transport channel than in the second dedicated transport channel.
In an advantageous embodiment, the coding and multiplexing stage is controlled to form a first symbol stream relating to a first dedicated physical channel based on the first dedicated transport channel, and at least one second symbol stream relating to a second dedicated physical channel based on at least one second dedicated transport channel.
Thus, the radio transmission stage can be so controlled that the radio signals transmitted have a first component along the first dedicated physical channel and a second component along the second dedicated physical channel, and that the first component has a greater transmission power than the second component. Another possibility, when the dedicated physical channels are multiplexed by a spread spectrum technique, is to control the coding and multiplexing stage and the radio transmission stage so that the first symbol stream has a smaller symbol bit rate than the second symbol stream, and that the first dedicated physical channel is associated with a higher spreading factor than the second dedicated physical channel.
Yet another possibility is to take advantage of the multiple receivers provided in certain radio communication terminals, especially when operated in macrodiversity mode, i.e. when the terminal communicates simultaneously with a plurality of base stations (see WO 00/38642). In the latter case, the radio signals can be transmitted from at least two base stations, the radio transmission stage being apportioned among said base stations, while arranging for the first and second symbol streams to be supplied to the radio transmission stage in distinct base stations, so as to form radio signals transmitted along different propagation paths.
In one embodiment of the method, the radio link control stage separately receives a first data flow belonging to a control plane, from which it forms the first data units, and at least one second data flow belonging to a user plane, from which it forms some at least of the second data units. The information of the specified type may then comprise radio resources control information and/or mobility management information and/or call control information.
In another embodiment, the radio link control stage receives a data flow belonging to a user plane, from which it forms the data units, while discriminating the first and second data units based on an analysis of said flow.
Other aspects of the present invention relate to an access network for a radio communication system and to a radio communication terminal, which comprise a radio link control stage, a medium access control stage, a coding and multiplexing stage and a radio transmission stage, which are arranged to implement a method of transmitting radio signals as defined hereinabove.